An angry letter - containing threats about shooting people in the face - was mailed to President Barack Obama, said New York City's police commissioner.

The US Secret Service confirmed that the White House intercepted the letter, which was similar to the ricin-laced notes sent last week to New York City Mayor Michael Bloomberg.

Police Commissioner Raymond Kelly said all the letters complained about gun control and said in so many words, "anyone who comes for my guns will be shot in the face".

The White House letter was seized at an off-site mail sorting facility and has been turned over to the FBI's Joint Terrorism Task Force for testing and investigation.

White House spokesman Josh Earnest said Mr Obama was aware of the letter addressed to him.

It comes after revelations that Mr Bloomberg had been targeted with two letters that have initially tested positive for traces of the poison ricin. They will undergo further testing.

The Sandy Hook shooting in December rekindled the gun-control debate

But Mr Bloomberg said he will not walk away from his campaign for tighter gun controls.

One letter was opened on Friday in New York at the city's mail facility and the other on Sunday in Washington DC at the headquarters of a nonprofit organisation Mr Bloomberg supports, Mayors Against Illegal Guns.

Both letters were postmarked from Louisiana and contained threats related to the gun-control debate and an oily pinkish-orange substance, the New York Police Department said.

Mr Bloomberg confirmed at least one of the letters "referred to our anti-gun efforts".

"But there's 12,000 people that are going to be killed this year with guns and 19,000 are going to commit suicide with guns, and we are not going to walk away from those efforts," he said.

Some officers who examined the letters in New York are showing symptoms of poisoning. However, civilian personnel in both locations who came into contact with the letters have not displayed symptoms.

The FBI Joint Terrorism Task Force and the NYPD division responsible for the mayor's protection are investigating the threats, police said in a statement.

Mr Bloomberg has been one of the country's most visible gun-control advocates since the December shooting of 20 children and six adults at a Connecticut school with a legally purchased, high-powered rifle.

Mr He helpsHeHe finances He finances the group Mayors Against Illegal Guns, which lobbies lawmakers and aims to counter the powerful US gun association.

Ricin is a poison found naturally in castor beans.

Several ricin-laced letters have been sent to US politicians in recent months, including a letter mailed to Mr Obama in mid-April.

A Mississippi man, James Everett Dutschke, is charged with making the poison and sending that letter, as well as letters to US Senator Roger Wicker and a Mississippi, judge.

An acquaintance of Dutschke's, Paul Kevin Curtis, was initially arrested in the case but later released. He reportedly claimed he had been framed, and named Dutschke as a possible suspect.